A projection exposure apparatus, in which a photosensitive substrate such as a wafer and a glass plate with a photoresist applied thereon is exposed with a pattern on a photomask or a reticle (hereinafter generally referred to as "reticle") through a projection optical system, has been hitherto used to produce, for example, semiconductor devices, liquid crystal display devices, and thin film magnetic heads by using the photolithography technique. Recently, the trend of technology is such that one chip pattern included, for example, in a semiconductor device becomes large. Accordingly, the projection exposure apparatus is required to handle a large superficial content of an objective transfer pattern so that the photosensitive substrate is exposed with a pattern on the reticle having a larger superficial content.
As the pattern of the semiconductor device or the like becomes finer and finer, the projection optical system is required to have an improved resolution. In order to improve the resolution of a projection optical system, the exposure field of the projection optical system may be increased, however, such an approach is difficult because of problems concerning design and production. Especially, when a catadioptric system is used as the projection optical system, an exposure field having no aberration is formed as a circular-arc shaped area in some cases.
In order to respond to the use of the large superficial content of the objective transfer pattern and the constraint of the exposure field of the projection optical system as described above, for example, a projection exposure apparatus of the so-called slit scan exposure system has been developed, in which a photosensitive substrate is exposed with a pattern having a superficial content wider than that of a slit-shaped illumination area on a reticle by synchronously scanning the reticle and the photosensitive substrate with respect to the illumination area having a circular-arc shape or a hexagonal shape (such an illumination area is hereinafter generally referred to as "slit-shaped illumination area").
The term "illumination area" herein means an area formed on a reticle illuminated with illumination light from an illumination optical system. The term "exposure area" herein means an area formed on a photosensitive substrate illuminated with light for exposure. The exposure area, which is formed as an area on the photosensitive substrate, is conjugate with respect to the illumination area through a projection optical system.
A projection exposure apparatus of the scan-and-stitch type has been proposed, in which the slit scan exposure system is adopted, however, a series of patterns are stitched and formed on a photosensitive substrate while partially superimposing a plurality of pattern images along a direction (referred to herein as "non-scanning direction", if necessary) perpendicular to a scanning direction. When the projection exposure apparatus of this type is used, a reticle is exchanged or the reticle is moved on a reticle stage in the direction perpendicular to the scanning direction after one time of scanning exposure, while a photosensitive substrate is moved in the direction perpendicular to the scanning exposure direction so that scanning exposure is performed two or more times. In general, the projection exposure apparatus is bound by definition of conditions for proper amount of exposure light and uniformity of illuminance with respect to a photosensitive material on a photosensitive substrate. Accordingly, the projection exposure apparatus of the slit scan exposure system also undergoes exposure light amount control in order that the exposure light amount for a photosensitive substrate is coincident with a proper exposure light amount within a predetermined allowable range, and the uniformity of illuminance for a wafer is maintained at a predetermined level. The exposure apparatus of the scan-and-stitch type described above is also required to undergo exposure light amount control in order that the uniformity of illuminance for a wafer is maintained at a predetermined level in the same manner as described above.
When the exposure apparatus of the scan-and-stitch type is used, a series of patterns are stitched and formed on a photosensitive substrate. Accordingly, there is a possibility that an area having a large cumulative exposure light amount and an area having a small cumulative exposure light amount may be locally generated at a stitching section on the photosensitive substrate. Such an inconvenience is caused because of the following reason. Namely, assuming that the light intensity rises from zero to 100% as if it follows a step function from the outside to the inside of a slit-shaped illumination area at both ends of the illumination area in the non-scanning direction (direction perpendicular to the scanning direction), the cumulative exposure light amount at the stitching section on the photosensitive substrate possibly becomes 2-fold or zero as compared with other areas if the positioning accuracy for the photosensitive substrate is low.
In order to countermove this inconvenience, Japanese Pat. Publication No. 46-34057 discloses a technique for stitching image planes (chip patterns) in the non-scanning direction by using a continuous luminescence light source such as a mercury lamp, in which the light intensity in the non-scanning direction of an exposure area on a photosensitive substrate is lowered at both ends thereof so that the light intensity distribution in the non-scanning direction is allowed to have a shape of an isosceles trapezoid. The technique for deforming the shape of illuminance distribution of the exposure area, which is described in the aforementioned official gazette, is also disclosed in Japanese Pat. Publication No. 53-25790 and SPIE, Vol. 774, p. 108 (1987) written by D. A. Markle.
Regardless of the exposure system such as the static exposure system and the scanning exposure system, other techniques for changing illuminance distribution of an exposure area include, for example, a method in which an illumination area is formed with defocusing by moving a field diaphragm along a direction of an optical axis as reported by J. P. Rominger in SPIE, Vol. 922, p. 188 (1988), and a method in which illuminance distribution in a direction perpendicular to a scanning direction is allowed to have a shape of an isosceles trapezoid by using an ND filter having linearly changing transmittance distribution as disclosed in U.S. Pat. No. 5,477,304.
In recent years, a plurality of circuit patterns are simultaneously provided on a reticle in order to improve the throughput by shortening the time required to exchange the reticle. A movable field diaphragm is used to select a desired transfer pattern area from such a plurality of circuit pattern areas on the reticle. The movable field diaphragm is usually arranged in the vicinity of a reticle pattern or at a position conjugate to the reticle pattern in an illumination system in order to sufficiently decrease the width of a shielding area around a pattern area intended for exposure. The superficial content of an opening formed by the movable field diaphragm can be changed in accordance with the size of the pattern area intended for exposure. For example, Japanese Patent Laid-open No. 7-94387 (U.S. patent application Ser. No. 08/255,997 filed on Jun. 8, 1994) discloses a movable field diaphragm having two movable blade (shielding plates) arranged in a scanning direction of a scanning type exposure apparatus. In this technique, an edge of each of the blades for defining an opening of the field diaphragm is moved during scanning exposure in synchronization with scanning movement of a reticle so as to cover a reticle pattern portion to be shielded other than portions defined as shielding bands on the reticle.
In the technique for changing the illuminance distribution of an exposure area in the non-scanning direction described above, patterns are stitched on a photosensitive substrate in the direction perpendicular to the scanning direction. In this procedure, the right and left sides of the light intensity distribution of the exposure area are symmetrical in the direction perpendicular to the scanning direction, and a stitching section receives the same amount of exposure light as that received at its opposite side. For example, the illuminance distribution for the non-scanning direction obtained by integrating the illuminance over the scanning direction has a shape of an isosceles trapezoid in the conventional technique described above. In such a conventional technique, the illuminance of exposure light also gradually decreases toward both ends in the non-scanning direction at the area located on the side opposite to the stitching section. Therefore, an exposed portion on the side, at which no pattern image is superimposed, undergoes exposure only once at a low light intensity, resulting in occurrence of under exposure as compared with a central portion. For this reason, a problem arises in that uniform exposure is not obtained over the entire photosensitive substrate. In order to avoid such local under exposure, it has been necessary to provide a shielding plate having a width wider than conventional one on the side at which no stitching section is formed.